Like all scientific mysteries, the Indian Ocean geoid low (IOGL) — also known as the Indian Ocean’s “gravity hole” — shouldn’t exist based on what we know. And yet, there it is. Fortunately, a team at the Indian Institute of Science published a seemingly sound explanation in May: the gravity in that spot is relatively low because down in the Earth’s mantle, underneath the Indian Ocean, hot, upward “plumes” or currents of rock altered the mantle’s structure.
The finding is more profound than it may seem — but to understand why, you first have to understand why the geoid matters in the first place.
The real shape of Earth
From a certain perspective, the geoid is the real shape of Earth.
Earth isn’t a perfect sphere, and we wouldn’t be able to, say, launch and use satellites if we wrongly acted as if it was. An ellipsoid — the shape of a yoga ball when you’re sitting on it — is a more accurate description of Earth’s overall shape. But to a rotating satellite, an ellipsoid isn’t quite right either. The actual shape in this context can only be seen in a representation of Earth’s geoid.
The geoid isn’t just a topographical map; it’s a map of Earth’s gravitational field at the surface. This is how an object orbiting Earth “experiences” our planet’s shape, if you will. It has peaks and valleys that mostly correspond to identifiable causes, and some are no-brainers — like mountain ranges or the deepest depths of the ocean.
Gravitational anomalies
But Earth’s geoid also has some weird spots that can’t be explained by anything as simple as topography, and there’s none weirder than the IOGL.
The big, dark blue blob is the IOGL
Credit: JPL / GRACE global gravity animation
Using “time-dependent global mantle convection models” of the Earth since the Cretaceous period, guided by our understanding of tectonic plate movements, the researchers deduced the cause of the IOGL’s formation by simulating the flow of heat over millions of years. Pieces of the Earth’s crust that sank over time caused hot rock in the mantle to rise below the Indian Ocean. The IOGL, then, is an area of low mass below that blob on the map — caused by the rising rock we mentioned earlier — and the resulting slight downtick in gravity.
As the paper phrases it: “sinking Tethyan slabs perturbed the African Large Low Shear Velocity province and generated plumes beneath the Indian Ocean, which led to the formation of this negative geoid anomaly.”
So what does all this mean about the true shape of Earth? “The Earth is basically a lumpy potato,” paper coauthor Attreyee Ghosh told CNN.
